1. Field of the Invention
The invention concerns silicon dioxide dispersions, their production and use.
2. Discussion of the Background
Aqueous silicon dioxide dispersions are used in polishing applications (CMP), in the paper sector (inkjet) or in glass production.
For economic and applicational reasons, the use of dispersions having a high content of silicon dioxide powder is desirable here. Economic can refer for example to the reduction of costs by transporting more highly filled dispersions. Such a dispersion can then be diluted to the desired content on site.
Furthermore, special applications demand highly filled dispersions. This applies for example to the production of glass articles. An aqueous silicon dioxide dispersion can be converted first of all into a green body which by means of further heat treatment, optionally with subsequent sintering, is converted into a glass body. The use of a highly filled dispersion reduces shrinkage during production of the green body and minimises cracking.
It is known from U.S. Pat. No. 4,042,361 that aqueous dispersions containing silicon dioxide produced by a flame hydrolysis process and no stabilisers display an acceptable stability only up to a fill content of up to 30 wt. %. With higher fill contents gelation or sedimentation can occur within a very short time.
U.S. Pat. No. 5,116,535 describes a process for producing a stable, aqueous dispersion containing at least 35 wt. % of silicon dioxide produced by a flame hydrolysis process and likewise no stabilisers. In this process silicon dioxide is introduced into water in a quantity that initially leads to a higher concentration in the dispersion than is desired. In a second step this predispersion is diluted with water to the desired concentration. The higher fill content achieved in comparison to U.S. Pat. No. 4,042,361 results from the higher viscosity of the predispersion, which increases the efficiency of dispersion. The disadvantage is that production of the dispersion involves two steps, and because of the high viscosity of the predispersion high dispersion energies are necessary.
U.S. Pat. No. 5,246,624 describes the production of a stabilised dispersion wherein silicon dioxide is introduced into acidified water in a concentration that is higher than desired. Acidification is preferably performed with mineral acids. The subsequent addition of base leads to a stabilisation of the dispersions in the alkaline pH range and the dispersion can be diluted to the desired concentration.
The disadvantage here is that production has to be started in the acid range. During the subsequent addition of base, the neutralisation causes salts to form, which can cause a disadvantageous change in the rheological properties of the dispersion.